Fat-burning zone explained: do you burn fat or carbs?

You burn both at every effort you can hold for long. As you work harder, the share of your energy that comes from fat falls and the share from carbohydrate rises. The amount of fat you burn behaves differently: it is tiny at rest, climbs to a broad peak at a moderate effort and falls away above it. So a high share of fat and a large amount of fat are different things. That pattern is consistent across the studies we read. Where your own peak sits is the uncertain part, because it differs widely from person to person.

It is a mix at every effort. In eight cyclists, fat supplied 56% of the energy at rest and still 24% at a hard effort.
Share and amount tell different stories. The share of fat was as high at rest as at any effort. The amount was not: 0.08 grams a minute at rest, against 0.68 to 0.79 at light and moderate efforts.
The peak is personal. In 1,121 athletes it sat anywhere between 22.6% and 88.8% of maximal oxygen uptake.
Training and time both move the mix. At about 200 watts, professional cyclists burned 0.65 grams of fat a minute and moderately active men 0.18. Over four hours at one steady effort, the mix drifted toward fat.
Hard efforts lean on the small tank. Glycogen, the carbohydrate you store, is about 4% of the body's fuel stores.

Somewhere on your watch, or on the console of a gym treadmill, there is a zone called fat burn. On Fitbit devices the description mentions a brisk walk.¹ It is easy to read that label as a promise: stay in here, because this is where you burn the most fat.

There is something real behind the label. At low efforts, fat does supply a large share of your energy. Whether that is also where you burn the most fat is a different question. To see why, it helps to look at what the body burns at every effort, and to count it in two ways.

One thing first. This piece is about fat as a fuel for moving. Losing body fat is a separate subject, and we leave it alone here.

1 Do you ever burn only one fuel?

No. Your muscles run on a mix of fat and carbohydrate, and what changes with effort is the proportion. A Dutch and British group showed this in 2001 with eight male cyclists, measured after a night without food: first at rest, then at three efforts of half an hour each.² The efforts were set at 40, 55 and 75% of each rider's maximal power, which came to 44, 57 and 72% of his maximal oxygen uptake, the ceiling we describe in our piece on VO2max. We will call them light, moderate and hard.

Sitting still, the riders got 56% of their energy from fat. At the light effort that was 55%, and at the moderate effort 50%. With only eight riders those three cannot be told apart: about half the energy came from fat, from the sofa up to a moderate effort. At the hard effort it had fallen to 24%, with carbohydrate supplying the other 76%, and that drop is clear (figure 2).

Four horizontal bars, each split into fat and carbohydrate. At rest: 56% fat, 44% carbohydrate. Light effort, 44% of maximum: 55% fat, 46% carbohydrate. Moderate effort, 57% of maximum: 50% fat, 51% carbohydrate. Hard effort, 72% of maximum: 24% fat, 76% carbohydrate. Eight male cyclists. — **Figure 2.** **It is a mix at every effort.** Published shares; rounding makes two of the rows add up to 101. With eight riders the first three rows cannot be told apart. Effort is given as a percentage of each rider's maximal oxygen uptake.

Nowhere on that scale does one fuel switch off and the other switch on. For you that means every ride or run draws on both, in a proportion that slides with how hard you go.

2 So where do you burn the most fat?

That depends on what you mean by most. Count the share, and low efforts win. The eight riders got as large a share of their energy from fat sitting still as at any effort on the bike. An older study, which included a much easier effort at a quarter of maximal oxygen uptake, found the mix richest in fat there.³

Count the grams, and low efforts lose. Sitting still, the eight riders burned 0.08 grams of fat a minute. At the light effort that was 0.68 grams and at the moderate effort 0.79, a difference too small to call with eight riders. At the hard effort it dropped to 0.51. Per hour that is roughly 5, 41, 47 and 31 grams. As a yardstick for the numbers that follow: half a gram a minute is 30 grams an hour. The older study saw the same in its own riders: they burned more fat at 65% of their maximum than at 25%, although the mix at 25% was the richer in fat.³

So a high share of fat and a large amount of fat do not belong to the same effort (figure 3). Much of the confusion around the fat-burning zone comes down to that difference.

Two panels over the same four efforts. Top, the share of energy from fat: 56% at rest, 55% at the light effort, 50% at the moderate effort, 24% at the hard effort; a bracket over the first three reads about the same share, sitting still or riding. Bottom, grams of fat burned per minute: 0.08 at rest, 0.68, 0.79 and 0.51; a bracket over the light and moderate bars reads most fat, too close to call between these two. — **Figure 3.** **Two questions about the same rides.** The share of fat was about the same at rest and on the bike. The amount was not: almost nothing at rest, and most at the light and moderate efforts, which are too close to tell apart with eight riders.

3 How can the same share be so much more fat?

Because the total grows. From the sofa to the light effort, the share of fat stayed where it was, 56 and 55%. What changed was the size of the pie: by our conversion the riders went from about 1.4 kilocalories a minute to about 12.2. The same slice of a far bigger pie is far more fat, 0.68 grams a minute where it had been 0.08 (figure 4).

Stacked bars of energy burned per minute in kilocalories, our own conversion. At rest 1.4 in total, of which fat 0.8. Light effort 12.2: fat 6.6, carbohydrate 5.6. Moderate effort 15.6: fat 7.7, carbohydrate 7.9. Hard effort 20.1: fat 5.0, carbohydrate 15.1. An arrow from the resting bar to the light bar reads same share, bigger pie. — **Figure 4.** **The same slice of a much bigger pie.** The kilocalories are our own conversion: the published grams multiplied by the energy per gram (40.80 kilojoules for fat, 16.19 for carbohydrate), divided by 4.184. It reproduces the published shares to within two percentage points.

One detail helps when you read grams. A gram of fat holds about two and a half times the energy of a gram of carbohydrate: 40.80 against 16.19 kilojoules.⁴ That is why 0.79 grams of fat and 2.04 grams of carbohydrate a minute came out at about half the energy each.

Above the moderate effort the picture changes. The pie keeps growing, but now the fat slice shrinks in size as well as in share, and carbohydrate covers all of the extra work. At the hard effort the riders burned 3.91 grams of carbohydrate a minute, a rate of about 235 grams an hour.

4 What does the whole curve look like?

The riders in that study stopped at a hard effort they could hold for half an hour. A 2018 study took 22 professional cyclists all the way to exhaustion, ten minutes at a time, and recorded both fuels at every step.⁵

Carbohydrate climbed the whole way, from 1.46 grams a minute at 137 watts to 6.02 at 372 watts, a rate of about 360 grams an hour. Fat rose to a peak of 0.67 grams a minute at 239 watts, about 40 grams an hour, and then fell away to 0.02 at 372 watts (figure 5). These are professionals, so the scale is theirs and not yours. What you can take from it is the shape, which a review describes in the same terms: fat burning rises from low to moderate efforts and falls when the effort becomes high.⁶

Two line charts against power output for 22 professional cyclists. Carbohydrate rises steadily from 1.46 grams per minute at 137 watts to 6.02 at 372 watts. Fat rises from 0.50 to a peak of 0.67 grams per minute at 239 watts, then falls to 0.02 at 372 watts. A final stage at 397 watts is drawn with open circles: 6.37 for carbohydrate and 0.00 for fat. — **Figure 5.** **The whole range, up to exhaustion.** Two different scales: carbohydrate runs to more than six grams a minute, fat stays under one. The open circles are the final stage, at 397 watts. That is above the group's average maximum of 379 watts, so not every rider reached it.

Read the far right of that curve with some care. Close to maximum, the breath measurements behind these numbers overstate carbohydrate and understate fat.⁷ Sources differ on where that starts: one puts the limit of reliable measurement at 80 to 85% of maximal oxygen uptake,⁷ another at 75%.⁸ The data can show fat burning fading to almost nothing. They cannot show that it stops.

Why it fades is less settled than the fact that it does. The 2001 study pointed to one step in particular, getting fat into the part of the muscle cell that burns it, slowed either by a shortage of the carrier that takes it there or by the muscle turning more acid.² A later review lists several possible brakes side by side, from fat leaving the body's fat stores more slowly to fat entering the muscle cell more slowly.⁹

5 Where does that peak sit?

This is where a single band runs into trouble. In the study that introduced the test for it, moderately trained cyclists reached their peak at 64% of maximal oxygen uptake on average, and individually anywhere between 42 and 84%.¹⁰ A second group of 55 trained men, also on bikes, averaged 62.5%.¹¹

Two larger studies on treadmills landed lower. In 300 healthy men and women the average was 48.3%, with individuals between 25 and 77%.⁷ In 1,121 athletes from a range of sports it was 49.3%, with the lowest individual at 22.6% and the highest at 88.8% (figure 6).¹²

Four rows on a scale of percentage of maximal oxygen uptake. Achten 2002, moderately trained cyclists: average 64%, individuals from 42 to 84. Achten and Jeukendrup 2003, 55 trained men: 62.5%. Randell 2017, 1,121 athletes: average 49.3%, individuals from 22.6 to 88.8. Venables 2005, 300 men and women: average 48.3%, individuals from 25 to 77. — **Figure 6.** **Where the peak sat, study by study.** Dots are averages. The bars run from the lowest to the highest individual, in the three studies that report them. In the first study a full curve could be drawn for 11 of the 18 riders.

The peak is also more of a plateau than a point. In the cyclists, fat burning stayed within 10% of its highest rate between 55 and 72% of maximal oxygen uptake.¹⁰

Researchers have tried to explain why people differ, mostly in how high their peak is. In the 300 men and women, lean body mass, activity level, aerobic fitness, sex and fat mass together accounted for 34% of the differences in grams per minute. Per kilogram of lean mass, three of them (fitness, activity and sex) accounted for 12%.⁷ In the 1,121 athletes, body composition and fitness left more than half unexplained.¹² With individual values that far apart, a group average tells you little about where your own peak sits.

Sex is one of the factors that does show up. In the treadmill study the women reached their peak at a higher effort than the men, 52 against 45% of maximal oxygen uptake.⁷

6 So is the zone on a watch wrong?

Not wrong. But it is a band, people are a spread, and the band itself depends on which watch you wear.

Fitbit defines its fat burn zone in two ways.¹ On the devices it lists by name, among them the Pixel Watch and the Charge, Sense and Versa series, the zone runs from 40 to 59% of your heart rate reserve, the gap between your resting and your maximum heart rate. On all its other devices it runs from 50 to 69% of maximum heart rate. In Fitbit's own example of the first kind, a 45-year-old with a resting heart rate of 74, the zone runs from 114 to 134 beats a minute, which is 65 to 77% of that person's maximum.

Now the measured peaks, in the same unit. The 300 men and women reached theirs at 61.5% of maximum heart rate on average, with individuals from 41 to 91%.⁷ The trained cyclists reached theirs at 74% on average, with individuals from 54 to 92% (figure 7).¹⁰

Four rows on a scale of percentage of maximum heart rate. Fat burn zone on Fitbit's listed devices, in the maker's example: a bar from 65 to 77%. Fat burn zone on all its other devices: a bar from 50 to 69%. General population, 300 men and women: average peak at 61.5%, individuals from 41 to 91%. Moderately trained cyclists: average peak at 74%, individuals from 54 to 92%. — **Figure 7.** **One maker's zone, in two versions, against two measured groups.** The bars are a maker's definitions, not measurements. The 65 to 77% is our conversion of the maker's own worked example for the devices it lists by name. On those the zone depends on your resting heart rate, so it differs from person to person; with a lower resting heart rate it sits lower.

As drawn here, each version of the band catches one of those two averages and misses the other. The reserve version also moves with the person: by Fitbit's formula, the lower your resting heart rate, the further the band slides back toward the other one. Neither version can catch the spread. Individual peaks ranged across 50 and 38 percentage points of heart rate in the two groups, and the bands as drawn are 12 and 19 wide. For you as an individual, then, the zone is a rough guess. The watch also has to work out your maximum heart rate first, which it does from your age with the common formula of 220 minus age.¹ We go into that in what your watch actually measures.

7 Does training change the mix?

Yes. The classic description of the fuel mix, from 1994, treats it as a tug of war: effort pulls toward carbohydrate, and endurance training pulls toward fat.¹³ Studies that follow the same people through a training programme find more fat in the mix afterwards at the same power output.¹⁴

A 2018 comparison shows how far apart two groups can be. Alongside the professionals it tested 20 moderately active men on the same protocol.⁵ At about 135 watts the professionals burned 0.50 grams of fat a minute and the moderately active men 0.38. At about 200 watts it was 0.65 against 0.18, or roughly 39 against 11 grams an hour (figure 8).

Two lines of grams of fat burned per minute against power output. Professional cyclists: 0.50 at 137 watts, rising to 0.67 at 239 watts and falling to 0.02 at 372 watts. Moderately active men: 0.38 at 132 watts, 0.31 at 165 watts, 0.18 at 198 watts and 0.09 at 231 watts. Later stages are drawn with open circles. At about 200 watts the two groups are marked: 0.65 against 0.18. — **Figure 8.** **The same power, very different amounts of fat.** Two groups measured once each, so this shows a difference between people, not a change within them. Open circles are stages above a group's average maximal power, 379 and 246 watts, which not every rider reached.

Part of that gap is simple. Two hundred watts is a little over half of what the professionals reached at the end of the test, 379 watts on average, and about four fifths of what the moderately active men reached, 246 watts. The same power is a lighter effort for the trained rider, and a lighter effort comes with more fat in the mix. For you that means the same pace or power can come with a different mix than it does for the rider next to you.

Whether training also changes the mix at the same fraction of your own maximum is less clear. A study of seven trained and seven untrained men found a difference in the mix only at easy efforts and none at a moderate one. Its hard effort showed no difference either, but there the authors doubt their own measurement, for the reason given in section 4.¹⁴ Another, with eight men in each group, found the peak of fat burning at a somewhat higher fraction in the trained men, 49.9 against 43.5%, and nearly twice as high in amount: 462 against 250 milligrams a minute, or 0.46 against 0.25 grams.¹⁵ The trained men in that study also had the larger aerobic capacity, so part of their higher peak is the bigger pie again.

What the 2018 comparison can and cannot show: the professionals and the moderately active men are two different groups, each measured once. The professionals were also younger, 26.8 against 40.0 years on average, so training is not the only thing that differs between them.

8 Does the mix change during a long session?

It does, slowly. Seven trained men set out to cycle for four hours at one steady, moderate effort after a light meal. Five completed it and two stopped after three and a half hours.⁴ The researchers followed the mix through the breath: the ratio of carbon dioxide breathed out to oxygen taken in, which falls as fat takes a larger share. It fell from 0.89 after half an hour to 0.83 after two and a half hours, and stayed low to the end (figure 9). In the paper's own chart, fat overtakes carbohydrate as the larger source of energy between two and two and a half hours in.

A line falling over four hours of cycling in seven trained men: 0.89 after 30 minutes, 0.88 after one hour, 0.86, 0.85, 0.83 after two and a half hours, 0.83, 0.82, and 0.81 after four hours. — **Figure 9.** **Four hours at one steady effort.** The breath ratio is carbon dioxide breathed out divided by oxygen taken in. A lower value means a larger share of fat. Five of the seven riders completed the four hours.

An analysis that pooled published cycling studies found the same direction: the longer the exercise, the lower that ratio.⁸ An older study of five trained cyclists adds a useful detail. Over two hours at a moderate effort, the totals from fat and from carbohydrate did not change. What shifted was where the fuel came from: less from the muscle's own stores, more from fat and sugar delivered by the blood.³ So on a ride or run of several hours, the mix you finish on is probably different from the mix you started on, and the change builds gradually.

9 Where does each fuel come from?

By more than one route. Some fuel is delivered by the blood, and some is stored in or near the muscle that is doing the work. The 2001 study could tell three sources apart: free fatty acids carried by the blood, sugar carried by the blood, and glycogen, the carbohydrate stored in the muscle itself. The rest of the fat, partly stored in the muscle and partly carried in the blood in bound form, it could only report as one group (figure 10).²

Three bars split in four. Light effort: free fatty acids from the blood 31%, other fat 24%, sugar from the blood 10%, glycogen in the muscle 35%. Moderate effort: 25%, 24%, 13%, 38%. Hard effort: 15%, 9%, 18%, 58%. — **Figure 10.** **Blood or muscle: where each fuel came from.** Shares of total energy as published. 'Other fat' is fat stored in the muscle plus fat carried in the blood in bound form; the study could not separate those two.

At the light effort, free fatty acids from the blood supplied 31% of the energy and muscle glycogen 35%. At the hard effort those two were 15% and 58%. For you that means the harder you go, the more of the work is paid for from a store that sits inside the working muscle itself.

10 Why does the small tank matter?

Because the fuel that carries hard efforts is the one you carry least of. On average the body stores about 500 grams of glycogen in the muscles and about 80 grams in the liver. Together that is about 4% of its fuel stores (figure 11).¹⁶ Fat sits at the other end of the scale: even someone with 7 to 14% body fat carries more than 30,000 kilocalories of it.¹⁷

A grid of one hundred squares, four of them blue. Glycogen is 4% of the body's fuel stores: about 500 grams in the muscles, range 300 to 700, and 80 grams in the liver, range 0 to 160. The other 96% is everything else; a lean person carries more than 30,000 kilocalories as body fat alone. At a hard effort 76% of the energy came from carbohydrate and 58% from glycogen in the working muscle. — **Figure 11.** **The small tank and the large one.** Glycogen amounts are averages with their normal range. The body-fat figure is for someone with 7 to 14% body fat. The hard-effort numbers are those of the eight cyclists in figure 2.

That mismatch is why carbohydrate is the fuel endurance athletes think about running out of, and why how much to take in during a long effort is a subject of its own. We will look at that separately. This piece stops at what is burned.

Who was measured: the detailed numbers here come from small groups of men on stationary bikes: 8, 22 and 7 trained cyclists, and 20 moderately active men. From the large mixed groups, 300 men and women and 1,121 athletes, this piece takes only where the peak sits and how high it is. For scale: across published cycling tests, peak fat burning averaged 0.53 grams a minute in endurance-trained men, 0.46 in recreationally active men and 0.35 in recreationally active women.¹⁸ One review also reports that fat burning is higher in running than in cycling.⁶

11 So what does this leave you with?

Three answers, because the fat-burning zone hides three questions.

Where is the share of fat highest? At low efforts, and it is as high at rest as anywhere. That much a low zone gets right.
Where do you burn the most fat per minute? At a moderate effort, on a broad plateau. Where it sits differs so much between people that an average, or a band on a watch, says little about you.
What carries a hard effort? Carbohydrate, most of it from a store inside the muscle that is small next to what you carry as fat.

None of this says which effort makes for better training. That is a different question with its own research, and this piece has not answered it.

A large share of fat and a large amount of fat are different things. On a hard day, what counts is the amount, and the size of the tank it comes from.

How we did this (for the curious)

This is an explainer, not a review, so there is no systematic search behind it. Before writing we drew up a scoping document from twelve sources that define the subject: what belongs in it, what does not, and where the sources disagree. We then read eleven more for specific questions, such as training, duration and fuel stores. Eighteen of the twenty-three are cited on this page.

Twelve of the eighteen we hold and read in full. Five we read as abstracts only, and from those we use only what the abstract states. Those are marked in the list below. Four of the full texts sit behind a paywall at the publisher and were read from archived copies of the publisher's own pages.

The fuel numbers come from breath: the oxygen a person takes in and the carbon dioxide they breathe out show which fuel is being burned, and two of the studies added labelled tracers to see where the fuel came from. We kept the method to the few sentences the figures need. One limit matters for reading them: close to maximal effort the method understates fat, so the right-hand end of figure 5 and figure 8 is the least certain part.

Several numbers are our own arithmetic. The grams per hour are the published grams per minute times sixty. The kilocalories in figure 4 are the published grams multiplied by the energy per gram given in one of the sources; they reproduce the published shares to within two percentage points, and where the 2001 study prints its own energy figures for fat (32 and 19 kilojoules a minute at the moderate and hard efforts) ours come to 32.2 and 20.8. The 65 to 77% of maximum heart rate for Fitbit's listed devices is our conversion of the maker's example. The comparisons of 200 watts with each group's maximum, the widths of the bands and of the individual ranges in section 6, and the milligrams turned into grams are ours as well. Watts are rounded to whole numbers.

In the 2018 study the last stages lie above each group's average maximal power, so not every rider can have reached them; the paper does not say how many did. We drew those stages with open circles and left them out of the text.

The studies did not feed their participants the same way, and that affects the mix. The eight cyclists and the eighteen cyclists were tested after an overnight fast, the 300 men and women after four hours without food, the professionals after an hour and a half, and the four-hour riders after a light meal. Numbers from different studies are therefore best compared for their shape, not gram for gram.

What we left out on purpose: low-carbohydrate and ketogenic diets, training on an empty stomach, how much carbohydrate to take in during a long effort, whether easy training has effects of its own, and weight loss. Some of those are subjects we intend to examine separately.

Sources

18 sources. 17 are research (T1) and 1 is a maker describing its own product (T3), which tells us what the product does and never how large an effect is. 12 of the research papers we hold and read in full; for the 5 marked Abstract we read the abstract at its own address and use only what it states. After each entry: what we took from it.

T3 Google Health Help Center (Fitbit and Pixel Watch). Track your heart rate with your Pixel Watch or Fitbit device. support.google.com/googlehealth/answer/14237928. Read 4 October 2026. A maker describing its own product: the fat burn zone as 40 to 59% of heart rate reserve on a named list of devices (where some label it the moderate zone) and as 50 to 69% of maximum heart rate on all other devices; the formula that turns reserve into beats; the brisk walk; the worked example of a 45-year-old with a resting heart rate of 74 and a maximum of 175, for whom the zone is 114 to 134 beats; and maximum heart rate from the common formula of 220 minus age.
T1 van Loon LJC, Greenhaff PL, Constantin-Teodosiu D, Saris WHM, Wagenmakers AJM. The effects of increasing exercise intensity on muscle fuel utilisation in humans. The Journal of Physiology. 2001;536(1):295–304. doi:10.1111/j.1469-7793.2001.00295.x. Eight male cyclists after an overnight fast, at rest and during three consecutive 30-minute stages at 40, 55 and 75% of maximal workload (44, 57 and 72% of maximal oxygen uptake): fat 0.08, 0.68, 0.79 and 0.51 grams per minute (56, 55, 50 and 24% of the energy), carbohydrate 0.15, 1.44, 2.04 and 3.91 grams per minute (44, 46, 51 and 76%); that the share of fat did not change from rest up to the moderate effort and that the rise in fat from the light to the moderate effort was not significant; the split by source in its Table 2; and its conclusion that the most likely mechanism at the hard effort is a slowing of fat transport into the mitochondria, through a fall in free carnitine or a fall in pH.
T1 Romijn JA, Coyle EF, Sidossis LS, Gastaldelli A, Horowitz JF, Endert E, Wolfe RR. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. American Journal of Physiology. 1993;265(3):E380–E391. doi:10.1152/ajpendo.1993.265.3.E380. Five endurance-trained cyclists at 25, 65 and 85% of maximal oxygen uptake: the breath ratio lowest at 25% (0.73, against 0.83 and 0.91), fat burning higher at 65% than at 25 or 85%; and over two hours at 65% the total rates of fat and carbohydrate burning unchanged, while reliance shifted from stores in the muscle to fat and glucose from the blood.
T1 Watt MJ, Heigenhauser GJF, Dyck DJ, Spriet LL. Intramuscular triacylglycerol, glycogen and acetyl group metabolism during 4 h of moderate exercise in man. The Journal of Physiology. 2002;541(3):969–978. doi:10.1113/jphysiol.2002.018820. Seven endurance-trained men cycling at 57% of maximal oxygen uptake after a light meal, five for the full 240 minutes and two until 210; the breath ratio at each half hour (0.89 at 30 minutes, 0.83 at 150, 0.81 at 240); fat burning higher and carbohydrate burning lower from 150 minutes on; its Figure 1, in which fat passes carbohydrate between 120 and 150 minutes; and the energy per gram used for our conversion, 40.80 kilojoules for fat and 16.19 for carbohydrate.
T1 San-Millán I, Brooks GA. Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and less-fit individuals. Sports Medicine. 2018;48(2):467–479. doi:10.1007/s40279-017-0751-x. Twenty-two male professional cyclists and 20 moderately active men, stages of about 35 watts every ten minutes to exhaustion; fat and carbohydrate burned at every stage (its Tables 2 and 3); age and maximal power of both groups (its Table 1).
T1 Achten J, Jeukendrup AE. Optimizing fat oxidation through exercise and diet. Nutrition. 2004;20(7–8):716–727. doi:10.1016/j.nut.2004.04.005. Abstract. That fat burning rises from low to moderate intensities and falls when the intensity becomes high, and that it is higher in running than in cycling.
T1 Venables MC, Achten J, Jeukendrup AE. Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study. Journal of Applied Physiology. 2005;98(1):160–167. doi:10.1152/japplphysiol.00662.2003. Three hundred healthy men and women on a treadmill after a four-hour fast; peak fat burning at 48.3% of maximal oxygen uptake (individuals from 25 to 77), equivalent to 61.5% of maximum heart rate (41 to 91); at 45% in men and 52% in women; five characteristics explaining 34% of the differences in peak fat burning, and three explaining 12% of it per kilogram of lean mass; and that close to maximum the breath method overestimates carbohydrate and underestimates fat.
T1 Rothschild JA, Kilding AE, Stewart T, Plews DJ. Factors influencing substrate oxidation during submaximal cycling: a modelling analysis. Sports Medicine. 2022;52(11):2775–2795. doi:10.1007/s40279-022-01727-7. Data from 434 published studies of cycling: that the breath ratio falls as exercise goes on, and that it is not reliable above 75% of maximal oxygen uptake.
T1 Spriet LL. New insights into the interaction of carbohydrate and fat metabolism during exercise. Sports Medicine. 2014;44(Suppl 1):S87–S96. doi:10.1007/s40279-014-0154-1. That several sites of down-regulation of fat burning may exist during high-intensity exercise, from the release of fat into the blood to its transport into the muscle cell and its mitochondria.
T1 Achten J, Gleeson M, Jeukendrup AE. Determination of the exercise intensity that elicits maximal fat oxidation. Medicine & Science in Sports & Exercise. 2002;34(1):92–97. doi:10.1097/00005768-200201000-00015. Eighteen moderately trained cyclists after an overnight fast, of whom 11 gave a full curve; peak fat burning at 64% of maximal oxygen uptake (individuals from 42 to 84), which corresponds to 74% of maximum heart rate (54 to 92); and fat burning within 10% of its peak between 55 and 72% of maximal oxygen uptake.
T1 Achten J, Jeukendrup AE. Maximal fat oxidation during exercise in trained men. International Journal of Sports Medicine. 2003;24(8):603–608. doi:10.1055/s-2003-43265. Abstract. Fifty-five endurance-trained men on a cycle ergometer; peak fat burning at 62.5% of maximal oxygen uptake, with a large spread between individuals.
T1 Randell RK, Rollo I, Roberts TJ, Dalrymple KJ, Jeukendrup AE, Carter JM. Maximal fat oxidation rates in an athletic population. Medicine & Science in Sports & Exercise. 2017;49(1):133–140. doi:10.1249/MSS.0000000000001084. Abstract. One thousand one hundred and twenty-one athletes on a treadmill; peak fat burning at 49.3% of maximal oxygen uptake on average, from 22.6 to 88.8% between individuals; more than half of the variation in peak fat burning not explained by body composition and fitness.
T1 Brooks GA, Mercier J. Balance of carbohydrate and lipid utilization during exercise: the "crossover" concept. Journal of Applied Physiology. 1994;76(6):2253–2261. doi:10.1152/jappl.1994.76.6.2253. Abstract. The fuel mix as the interaction of exercise intensity, which increases carbohydrate use, and endurance training, which promotes fat use.
T1 Bergman BC, Brooks GA. Respiratory gas-exchange ratios during graded exercise in fed and fasted trained and untrained men. Journal of Applied Physiology. 1999;86(2):479–487. doi:10.1152/jappl.1999.86.2.479. That studies following the same people show more fat in the mix after endurance training at the same absolute power; and its own comparison of seven trained and seven untrained men at the same relative efforts: a difference in the mix at 22 and 40% of maximum, none at 59 and 75%.
T1 Nordby P, Saltin B, Helge JW. Whole-body fat oxidation determined by graded exercise and indirect calorimetry: a role for muscle oxidative capacity? Scandinavian Journal of Medicine & Science in Sports. 2006;16(3):209–214. doi:10.1111/j.1600-0838.2005.00480.x. Abstract. Eight trained and eight untrained men, with maximal oxygen uptake of 4.6 and 3.5 litres a minute: peak fat burning of 462 against 250 milligrams per minute, at 49.9 against 43.5% of maximal oxygen uptake.
T1 Murray B, Rosenbloom C. Fundamentals of glycogen metabolism for coaches and athletes. Nutrition Reviews. 2018;76(4):243–259. doi:10.1093/nutrit/nuy001. Glycogen in muscle 500 grams on average (normal range 300 to 700) and in the liver 80 grams (0 to 160); together 4% of the body's total fuel stores.
T1 Purdom T, Kravitz L, Dokladny K, Mermier C. Understanding the factors that effect maximal fat oxidation. Journal of the International Society of Sports Nutrition. 2018;15:3. doi:10.1186/s12970-018-0207-1. That a person with 7 to 14% body fat has more than 30,000 kilocalories stored in fat tissue.
T1 Maunder E, Plews DJ, Kilding AE. Contextualising maximal fat oxidation during exercise: determinants and normative values. Frontiers in Physiology. 2018;9:599. doi:10.3389/fphys.2018.00599. Pooled values from published cycling tests after an overnight fast: peak fat burning of 0.53 grams per minute in endurance-trained men, 0.46 in recreationally active men and 0.35 in recreationally active women.

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This is educational material about which fuels the body uses during exercise. It is not training, nutrition or medical advice, and not a recommendation for or against any method, product or diet.

Fat, carbs and the fat-burning zone